The present invention relates to a method and apparatus for severing bar-shaped workpieces, such as ingots made of silicon, ferrite or the like, by cutting tools with a high degree of accuracy while the workpieces are rotated.
Conventionally, it is known to feed a rotatably fed bar-shaped workpiece by means of a grinding disk, wherein a ring-shaped grinding disk is defined along the inner periphery of a doughnut-shaped blade to make the outer periphery of the bar-shaped workpiece contact the inner periphery of this grinding disk. The workpiece is fed rotationally, while the grinding disk is rotated for severing wafers from the workpiece. This method is useful for reducing the curvature of the severed surface and improving the cutting efficiency. As shown in a paper titled Improved Slicing and Orientation Technique for I.D. Sawing, Industrial Diamond Review, issued Nov. 1977, pp. 385 to 387, since it is necessary to position the central axis of the grinding disk parallel to the rotational axis of the workpiece and to align the rotational center of the workpiece with the axis of the workpiece, the workpiece cannot be severed in such a way that the axis of the workpiece may be inclined relative to the axis of the grinding disk. Furthermore, since the cut end portion of the workpiece is not supported during the severing process, the unprocessed portion toward the rotational center portion which connects the main portion of the workpiece with the cut end portion is likely to be broken just before the completion of severing due to the action of the gravitational force or the like and causes defects on the processed portion in the form of protrusions or recesses. While the conventional method of severing the rotating workpiece by means of a grinding disk has advantages, it is not free from the above-discussed disadvantage. This makes it impossible to apply the method to an ingot of silicon or the like which must be severed by the grinding disk by inclining the ingot at an angle of 0.degree. to 7.degree., so as to be free from protrusions or recess around the central cut portion.